Our house is in western Washington and during the winter we get a lot water in the crawl space. I'd like to tackle installing a drainage system – probably with a sump pump – and then encapsulate the crawl space to warm up the floors above and control moisture from the crawl space.

I'm thinking that the approach would be to install a drain around the perimeter of the crawl space that runs back to a sump (or sumps) and pump from there. I think my idea is similar to How should I install a curtain drain in my crawl space? and the follow ups Curtain drain in the crawl space - Part 2 and Curtain drain in the crawl space - Part 3. But I have a few questions about the design, details, and materials:

  1. First is a design question, assuming that there isn't obvious evidence of a spring will a drain around the perimeter catch most or all of the water intrusion?

  2. How far away from the stem walls should I keep the trench for the drain?

  3. Is a "perforated drain pipe with sock" like this product from Home Depot a good choice for the drain or would I do better with one of the "drainage systems" that seem to the rage with the crawl space encapsulation specialists?

  4. Assuming that the pipe and sock is the right material, is there a rule of thumb for how large the trench should be?

  5. The pipe seems pretty flexible, so I'm wondering about keeping a constant grade. Do I want to do something like lay down a bed of sand and slope that to ensure drainage? Or is an overall pitch all I need?

  6. Would there be any reason not to use the material excavated from the trench to fill in low areas in the crawl space? I was thinking of trying to create either a flat floor or a gentle crown in the center.

  7. Is there anything else I should be thinking about?

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    What about redoing the surface contours around the house to channel surface water away from the house? After every heavy rain a pier and beam house I rented for a year in Dallas TX got a lake under it. This had to be due to the surface drainage paths being inadequate. For 38 years we have lived in a neighborhood of tract houses on slab foundations, many of which, including ours, have a sunken living room. Our sunken room is on the downhill side, but of those with it on the uphill side, some have had water ingress. Those that recognize the cause seem to prefer underground drains ($$$ !!!). Commented Dec 24, 2016 at 11:52
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    I stopped the water ingress on one house on the next street using a mattock, a shovel, and a wheelbarrow. Anytime I need exercise I go over there and move more soil from the uphill side to the downhill side. The owner keeps a natural yard with wildflowers and so there is no turfgrass to deal with. I understand that one can reestablish a swale by cutting out turf, setting it aside, digging out under-soil and replacing the turf, although this does not sound like fun to me. Commented Dec 24, 2016 at 12:02
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    In my locality the surface is a clay soil which sheds surface water. I can see that a full basement (or even a low crawl space) in highly permeable soil would present problems that would go beyond surface contouring. Commented Dec 24, 2016 at 13:11
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    So in your case the water table is rising in winter to a level at or above the level of the soil inside your crawl space. And the only solution would be to have pumped drains inside, and maybe also outside, the perimeter beam, right? And you are asking for information on how to do this. Commented Dec 24, 2016 at 17:18
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    Makes sense to me. If the water is moving in from below and laterally, and not from percolation through the soil right by the house, then regrading would do nothing to stop it. Another possibility sometimes mentioned is a seasonal spring under or near the house. One way to test the situation might be to drill small boreholes (for convenience on the outside of the perimeter beam, one per side) down to say 4 to 6 ft below the level of the soil in the crawlspace and see how the water level changes as the seasons change.Compare the level in the different wells. Commented Dec 24, 2016 at 20:40


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